Non-invasive Scattering Imaging Methods Based On Optical Memory Effect

Due to the inhomogeneity of the refractive index inside some samples,e.g.biological tissue and fog,or on the surface of some other samples,e.g.ground glass and rough wall,light will be strongly scattered when it propagates through or it is reflected by such samples.Therefore,it is always very difficult to obtain the information of the objects hidden behind the scattering media by using the conventional optical imaging system.Instead,only a complex speckle pattern,which is caused by the local interference of scattered light,will be acquired.Although the wavefront shaping technique has been demonstrated that it can successfully overcome the light scattering and realize focusing or imaging through scattering media by controlling the wavefront of the incident light,there are still some inevitable limits of this technique.Firstly,the feedback modulation process or the calibration process,e.g.measuring the transmission matrix the system,are very complex and time-consuming,which determines that it is almost impossible to use it in some real applications that involve the dynamic scattering media,e.g.the biological imaging;secondly,the optical or acoustical guide-stars in the planes of interest inside or behind the scattering media are always required.This fact also limits its effectiveness in some real applications.As early as 1988,the researchers found a very important feature of the thin scattering layers and named it as "optical memory effect":when the incident light is rotated within a small angle,the intensity distribution of the speckle pattern would not change obviously,instead,it would only be translated over a distance,which relates to the rotation of the incident light.Furthermore,a breakthrough of non-invasive imaging through scattering layers was put forwarded by exploiting the optical memory effect,and then a real-time scattering imaging method based on the same concept was also developed.Although the current methods have the potential to realize the non-invasive imaging through the dynamic scattering media,there are three problems to be further solved:i)the phase-retrieval algorithm is required in the current imaging method to recover the lost phase information,however,it is hard for the phase-retrieval algorithm to get the accurate Fourier phase of the hidden object,due to the inherent limits of phase-retrieval algorithm;ii)The point-spread-function(PSF)is always very important for an optical imaging system.However,the current imaging method cannot access the PSF of a highly scattering imaging system,since it is based on the statistical average of the sufficient speckle grains and the PSF will be removed during the reconstruction process;iii)The resolution of a highly scattering imaging system is still limited by the diffraction-limit,which determines any frequency that is higher than the limit of the system itself can hardly be acquired.In order to solve the aforementioned problems of current imaging methods based on optical memory effect,some imaging methods are proposed in this dissertation and they are briefly described as follows:(1)Firstly,it is experimentally demonstrated that the optical memory effect based single-shot speckle-correlation method can be used to observe the gray-scale objects,due to the fact that the autocorrelation of an intensity object contains the information of its intensity distribution.Secondly,it is experimentally shown that the scattering media can be used as a special "scattering lens".Compared with a conventional imaging lens,the"scattering lens" allows us to observe the objects that are hidden by a totally opaque barrier,which is placed at the light path,since the direction of scattered light is random and divergent.(2)We propose a non-invasive speckle imaging method based on the bispectrum analysis,and by using numerical simulations and experiments,it is shown that both the Fourier amplitude and Fourier phase can be independently extracted from a single high-resolution speckle pattern.Compared with the iterative phase-retrieval algorithm,the phase information extracted from the bispectrum are more accurate,since the bispectrum of the object contains the object's true Fourier phase.With the phase information extracted from the bispectrum analysis,we can recover the orientation of the object,which it is hard to be realized by phase-retrieval algorithm.Moreover,the process of extracting the phase information from the bispectrum is deterministic,which determines that it does not need to repeat the independent runs or require many iterations.Finally,this method performs better than phase-retrieval algorithm in noisy environments.(3)A non-invasive imaging method based on multiple speckle patterns and phase-diversity is proposed.By using numerical simulations and experiments,we show that the image of hidden object can be recovered,and simultaneously,the PSF of the highly scattering imaging system can be estimated.This phase-diversity based method has two advantages,firstly,only a small region of speckle patterns is sufficient to recover the hidden object,since the method is not based on the statistical average of large amounts of speckle grains,as is used in the speckle-correlation method;secondly,when the PSF is known,other hidden objects can be directly recovered by using a simple deconvolution method,instead of using other complex methods.(4)We propose a super-resolution scattering imaging method based on the multiple speckle illuminations.The image of the hidden object recovered by current speckle-correlation method is diffraction-limited and it means any frequency higher than the limit determined by the scattering imaging system cannot be recovered.In this method,we use the speckle patterns as the illumination sources and by calculating the high-order cumulant of multiple speckle patterns containing the information of the hidden object,we can theoretically observe the object with a resolution ?n times higher than the diffraction-limit,where n is the value of the order.Moreover,calculating the high-order cumulant of multiple speckle patterns can also suppress parts of the optical speckle pattern and increase the contrast between different speckle grains.This phenomenon allows us to directly observe the hidden objects without any reconstruction of speckle patterns.